1. Field of the Invention
This invention relates to scanning antennas. More specifically, this invention relates to dual reflector scanning antenna arrangements.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Antenna arrangements for scanning a beam in a single dimension across a field-of-view are currently used in a variety of applications, including satellite communication and automotive radar. In perhaps the simplest scanning arrangements an antenna assembly is rapidly rotated through a beam scan angle defining the field-of-view. Unfortunately, such single antenna systems typically manifest a relatively high moment of inertia, and hence require a rugged and powerful rotary joint drive mechanism to effect scanning at a sufficiently high rate. In addition, rotating an entire antenna having a high moment of inertia throughout a field-of-view may induce substantial vibration--a clearly undesirable phenomenon in the presence of other sensitive hardware.
Dual reflector antenna systems constitute an alternative means of effecting linear scanning of an antenna beam. In dual reflector systems, an antenna feed emits radiation which is reflected by a subreflector to a main reflector. The main reflector then projects the incident radiation from the subreflector as an antenna beam. The beam is then scanned over the field-of-view by translating the antenna feed relative to the subreflector.
In Cassegrainian dual reflector systems each reflector is constrained to be symmetrical about its own centerline, with the main reflector defining a paraboloid and the subreflector defining a hyperboloid. However, Cassegrainian systems having purely conic (paraboloid and hyperboloid) reflectors engender coma aberration (i.e. the appearance of particular sidelobes in the scanned antenna beam pattern as the antenna feed is moved back and forth).
Certain dual element antennas utilizing reflectors which depart from strictly conic surfaces have been devised to minimize coma and spherical aberration. For example, in Schwarzschild antennas the paraboloid and hyperboloid surfaces of a Cassegrainian antenna are perturbed in order to reduce the magnitude of coma lobes in the antenna pattern. A limited beam scan may be obtained using a Schwarzschild system by moving the antenna feed back and forth through a region of space approximating a focal plane. However, conventional Schwarzschild systems are not disposed to project a scanned antenna beam from a fixed feed location. Thus, Schwarzschild systems require a complex rotary joint mechanism to enable translation of the antenna feed.
In a particular dual element system disclosed by C. A. Rappaport, "An Offset Bifocal Reflector Antenna Design for Wide-Angle Beam Scanning", IEEE Transactions on Antennas and Propagation. Vol. AP-32, No. 11, Nov. 1984, pp. 1196-1204, both reflectors are fixed and are specially shaped to produce a pair of focal points. However, in order to utilize the system of Rappaport to generate a scanned beam the antenna feed would again need to be moved relative to the subreflector. In the Rappaport system this translation would occur along the contour of best focus between the focal points, and would be required to take place over an angle larger than the beam scan angle. A further disadvantage of the dual element arrangement disclosed by Rappaport is that a rotary joint would again need to be used to displace the antenna feed throughout the focal plane. Moreover, the translated feed assembly may also possess a moment of inertia of sufficient magnitude to cause undesired vibration.
Accordingly, a need in the art exists for a dual reflector antenna system having a scanning element characterized by a low moment of inertia, in which the scanning element is not required to scan an angle as large as the beam scan angle.